1. Field of the Invention
The invention relates to a device and method for forming solutions. In particular, it relates to a diffusional burette for controlled delivery of chemicals and biochemicals into a target material, such as a liquid, and will be described with particular reference thereto.
2. Discussion of the Art
Currently available methods for reagent and solution preparation involve the use of traditional tools, such as analytical balances and glassware, including beakers of different sizes, pipettes, and burettes.
Such tools are suitable for preparing reagents and solutions of relatively large volumes, in the milliliter range. However, a demand has arisen for delivery devices which are capable of delivering liquid volumes which are up to several orders of magnitude smaller, to reduce waste and associated costs, and also to accommodate the ever increasing sensitivity of the instrumentation techniques that use these reagents and solutions. To satisfy the emerging needs for solution preparation in the microliter (μL) volume range, pipettes based on liquid displacement by a precisely controlled volume of air have been introduced. For example, Eppendorf-type pipettes which use air/gas pressure and air/gas displacement for volumetric reagent transfer and delivery are now in use.
These devices deliver fixed or adjustable volumes of aqueous reagents from disposable plastic pipette tips by aspirating an appropriate volume of source solution into the tip and then delivering it into a target solution by reversing the air flow. A piston-type arrangement, inside the pipette body, whose air volume is precisely controlled, is used to meter the liquid volume that is aspirated into the tip, and subsequently delivered in one shot into the target solution.
For continuous (as opposed to bolus type) reagent delivery, mechanized piston burettes have been introduced, where a stepping motor controls the reagent volume aspirated, as well as delivered. Such mechanized burettes cover the microliter (mL) as well as μL ranges in terms of delivered volume. For delivering sub-μL volumes, different mechanized schemes have been conceived, such as a vibrating cantilever that “shoots” nanoliter (nL) droplets into the target across air.
While such novel pipette and burette designs reach into the μL volume ranges, the mechanical working principles do not facilitate their adaptation to the handling of even smaller target liquid volumes or smaller reagent increments delivered. One constraint for the pipettes is evaporation, which becomes significant, even during short periods of time, for droplets smaller than about 1 μL. Adjusting sub-μL air volumes accurately and precisely is also difficult. Moreover, dislodging a nL-size droplet from a pipette tip is often difficult, since capillary forces become stronger relative to droplet mass as the droplet size decreases. Thus, accuracy and precision for the delivery of sub-μL volumes by a pipette based on air displacement is not readily achieved.
Furthermore, preparation of solutions of relatively low concentrations often involves multiple steps that are difficult to perform with high final accuracy and precision. The first step is, typically, weighing a very small mass of solid (crystalline or powdered) material, i.e., the chemical that is to be present in the final solution, on an analytical balance. This, often tiny, amount of material may be hygroscopic (i.e., it absorbs water from humidity in air). This tends to falsify the weighed amount. Additionally, a powdery material may tend to float in air, contaminating the balance and the environment. The chemical may also be hazardous, posing problems to the user. The weighed material is then transferred into a beaker. These steps, requiring utmost care, are often sources of significant errors that propagate through all subsequent steps. Typically, multiple dilutions follow, until the desired low concentration is achieved. Further errors tend to be added in each step. In addition, such a procedure is labor intensive and prone to mistakes. Moreover, the process of serial dilution frequently uses far more of the raw material than is needed in the final solution than is needed, resulting in wastage of often expensive materials or hazardous waste production.
The mechanized burettes suffer similar drawbacks when sub-μL volumes are to be delivered in a continuous fashion. Accuracy and precision of the displaced reagent volume become worse as the delivered volume decreases, especially in the nL volume range. Parasitic diffusion between the burette tip and the target liquid may add errors that are difficult to estimate, and even more difficult to correct. On the other hand, if the burette tip is not in direct contact with the target liquid, droplets form at the tip that must be dislodged to reach the target. Thus, delivery by the burette becomes effectively discrete.
The present invention provides a new and improved reagent delivery device and method of use and fabrication which overcome the above-referenced problems and others.